CHICAGO (AP) — A milder type of mental decline that often precedes Alzheimer”s disease is alarmingly more common than has been believed, and in men more than women, doctors reported Monday.

Nearly a million older Americans slide from normal memory into mild impairment each year, researchers estimate, based on a Mayo Clinic study of Minnesota residents.

That”s on top of the half million Americans who develop full-blown Alzheimer”s or other forms of dementia — a problem sure to grow as baby boomers age. The oldest boomers turn 62 this year.

“We”re seeing that in fact there”s a much larger burgeoning problem out there” of people at risk of developing dementia, said Dr. Ronald Petersen, the Mayo scientist who led the study.

Dr. Ralph Nixon, a New York University psychiatrist and scientific adviser to the Alzheimer”s Association, was blunt.

“We”re facing a crisis,” he said.

There are no treatments now to prevent this mental slide or reverse it once it starts.

But that may be changing. Researchers on Monday reported early, somewhat encouraging results from an experimental nose spray that seemed to improve certain memory measures in a study of mildly impaired people.

The drug, for now just called AL-108, needs testing in a longer, larger study. It is being developed by Allon Therapeutics Inc., based in Vancouver, B.C.

Doctors said it shows the potential for new types of medicines that target the protein tangles that kill nerve cells, instead of targeting the sticky brain deposits that have gotten most of the attention up to now.

The studies were reported at the International Conference on Alzheimer”s Disease in Chicago.

Petersen is the scientist who defined mild cognitive impairment, or MCI, as a transition phase between healthy aging and dementia. It is more than “senior moments” like forgetting where you parked the car, but not as severe as having dementia, where you forget what a car is for.

People with it have impaired memory but not other problems like confusion, inattention or trouble putting thoughts into words.

The Alzheimer”s Association says more than 5 million Americans have Alzheimer”s, but no estimate for this “pre-dementia” has been available until now.

Petersen”s federally funded study involved roughly 1,600 people, ages 70 through 89, living in Olmstead County, which surrounds the Mayo Clinic in Rochester, Minn. All tested normal when they were enrolled in the study, but more than 5 percent had developed mild impairment when evaluated a year later.

Men were nearly twice as likely as women to develop it. That”s a surprise, because some studies have found more women with Alzheimer”s than men. But there may be a simple explanation:

Even though more men may be impaired, women outlive them and therefore have more time to develop full-blown dementia.

“This is a very large and important issue for our country and for the world,” said Duke University psychologist Brenda Plassman. A smaller study she published earlier this year backs up the Mayo study”s findings.

The mild impairment rate is two to three times larger than many researchers had expected, Petersen said.

“It”s the iceberg under the tip,” agreed Dr. R. Scott Turner, incoming director of the memory disorders program at Georgetown University Medical Center. A prime goal is finding drugs to treat the mild impairment before Alzheimer”s develops.

The AL-108 study tried to do that. Scientists gave 144 people with mild impairment either a low or high dose of the drug or a dummy drug for 12 weeks. The study missed its main goal — a composite of various memory scores — and the low dose showed no effect. But those on the higher dose improved on some memory tasks after one month and benefits lasted a month after they stopped treatment, said the study”s leader, Dr. Donald Schmechel of Duke University.

The study was sponsored by the drug maker.

In another study presented at the conference on Sunday and published on the Internet by the British medical journal The Lancet, researchers reported that dementia rates in developing countries may be considerably higher than official estimates and closer to rates in wealthy countries.

Scientists used a more liberal definition of dementia more suitable to poorer, less educated populations, where respect for family often means relatives don”t regard dementia as a burden so much and may be less likely to report problems.

The study involved nearly 15,000 people in 11 sites from China, India, Cuba, Mexico and other nations. Dementia rates ranged from nearly 6 percent in rural China to nearly 12 percent in the Dominican Republic, said co-author Martin Prince of King”s College in London.

The World Health Organization and the Alzheimer”s Association were among the study”s sponsors.